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Old 08-06-2010, 12:12 AM   #1
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wire size for conduit run


Hi evereryone, I am planing on running aproximatly 50 feet of wire in conduit to power two 240 volt pumps, according to the labels on the motors one draws 21 amps and one 26 for a total of 47 amps. I plan on using 4 wires as some of the controls are 120 volt. I plan on using thhn wire runing of a 60 amp double pole fuse approximatley 25 feet thru a basement in liquid tite flexible metalic conduit and then continuing outside underground in conduit for approximatley 25 feet. Dose this all sound good? Now for the questions
-What size wire should be used?(the ambiant temperature gets up to about 100 degrees fahrinheit)
-Can I use the liquid tite flexible metalic conduit for the under ground run?( part of it will be above ground and exposed to sunlight)
-And what size conduit would I need ? Thanks for your help.


Last edited by WDR; 08-06-2010 at 12:39 AM. Reason: forgot something
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Old 08-06-2010, 01:26 AM   #2
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wire size for conduit run


Unless someone on here has all the charts for the calculations involved you should probably have an electrical contractor do the installation. You need the starting amps for each motor and then need to do a voltage drop calculation for the distance involved just to size the wiring. Each of those motors should be on separate breakers or fuses sized for that motor. I did these calculations 35 years ago but would never try to do so today.

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Old 08-06-2010, 08:33 AM   #3
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wire size for conduit run


Where are you located ?
What kind of pumps...for a pool ?
50' is not that far, I doubt voltage drop will come into play
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Old 08-06-2010, 10:49 AM   #4
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wire size for conduit run


I will have each of the motors on a seprate breaker on the end of the run the pumps ar for irrigation and the run may actually be less than 50 feet probobly more like 40. I asked about this because I do not have the charts but I am aware of voltage drop and other similer factors.The only amp ratings I have are the tags on the pumps. I am located in murphysboro, IL. thanks again!
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Old 08-06-2010, 11:31 AM   #5
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wire size for conduit run


My pool pump has a MAX rating of 18.6a @ 120v, 9.3A @ 240v

If that is the Max amp rating on the pumps then you need #10 wire & a 30a breaker for each

You need THWN for wire for outside
50' is not a problem for voltage drop

Does the pump have a place for 4 wires ?
Or are you going to run a sub-panel & put breakers in to power pumps & other 120v loads ?
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Old 08-06-2010, 11:44 AM   #6
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wire size for conduit run


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Originally Posted by WDR View Post
Hi evereryone, I am planing on running aproximatly 50 feet of wire in conduit to power two 240 volt pumps, according to the labels on the motors one draws 21 amps and one 26 for a total of 47 amps. I plan on using 4 wires as some of the controls are 120 volt. I plan on using thhn wire runing of a 60 amp double pole fuse approximatley 25 feet thru a basement in liquid tite flexible metalic conduit and then continuing outside underground in conduit for approximatley 25 feet. Dose this all sound good? Now for the questions
-What size wire should be used?(the ambiant temperature gets up to about 100 degrees fahrinheit)
-Can I use the liquid tite flexible metalic conduit for the under ground run?( part of it will be above ground and exposed to sunlight)
-And what size conduit would I need ? Thanks for your help.
Sounds like your wiring a feeder on a 60 amp breaker to a pump controller then 240 volts from the controller to circuit breakers protecting each motor is that correct?
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Old 08-06-2010, 11:44 AM   #7
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wire size for conduit run


I plan on installing 4 wires running to the pumps with a control panel the only load on this panel would be the pumps and some 110 controlls I can run either of the pumps of of a set of 30 amp fuses with no problems but I have to run one 60 amp to the controll panel because the only oponing in the 200 amp main fuse panel is for 2 60 amp fuses. one of my concerns was that I do not know the temp rating of the fuse box and would have to derate the thhn and use thicker wire.each pump would have its own protection at the control panel

Last edited by WDR; 08-06-2010 at 11:54 AM. Reason: mistake
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Old 08-06-2010, 11:46 AM   #8
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wire size for conduit run


My questions where
-what size thhn hots, neutral, and ground do I need taking into account the old fuse box, the conduit, and that it gets up around 100 degrees fahrenheit outside in the sun
- can I use the metalic water tite flex under ground and what size do I need.

Last edited by WDR; 08-06-2010 at 12:01 PM. Reason: mistake
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Old 08-06-2010, 12:11 PM   #9
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wire size for conduit run


Ok I think I understand what your doing now. To get this right there are a few calculations that may take me some time maybe 30 minutes or so.

Do you know the horsepower of each motor?
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Old 08-06-2010, 12:29 PM   #10
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wire size for conduit run


Ok I will go over the whole thing again starting at a 200 amp fuse panel with a double 60 amp fuse in the basement run 25 feet thru flexible liquid tite metalic conduit go under ground for about 15 feet then to the controll panel with 2 30 amp fuses for each pump.some other details below
-the pumps are each 5 horse power and either one runs with no problems on a 30 amp circuit.
- 220 with 4 wires as some of the controlls use 110
-the wire will be type thhn(this wire is also rated as thwn and mtw)
now the questions again.
- what size hot neutral and ground wires are needid taking into acount the fuse box with a unknown temp rating, the conduit, and that the ambiant temputure exceeds 30 degrees Celcius( it gets to about 100 degrees F in the summer)
- can the flex conduit go under ground and what size do I need. thanks for everyones help!
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Old 08-06-2010, 01:02 PM   #11
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wire size for conduit run


Conductor size and feeder protection are taken from tables that show an flc for a specific horsepower not what is on the motor nameplate. Which is why I asked for the horsepower of the motors.

We can use the motor nameplate fla and get pretty close and I am assuming your motors have overload protection located internal to the motor and the 30 amp fuses (not circuit breakers) are for overcurrent and ground fault protection.

If you have the horsepower of the motors it would be a better calculation.

Back after lunch .. post the hp if possible
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Old 08-06-2010, 01:09 PM   #12
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I am very sorry about the horse power, they are both 5 hp however the maximum main i can put in is a 60 amp.you are correct on your other asumptions.

Last edited by WDR; 08-06-2010 at 01:11 PM. Reason: mistake
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Old 08-06-2010, 02:58 PM   #13
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wire size for conduit run


Not a problem. If we use the motor tables for 5 hp your flc (full load current) is 28 amps for both motors.

Lets do the branch circuit circuit conductors from the controller and the fuse blocks first.

For branch circuit conductor size the calculation is ... 1.25 x 28 = 35 amps minimum this would be a #10 thwn at 75C or a #8 thwn at 60C terminations installed in conduit. You must assume 60C if that is unknown.

The fuses in the controller fuse blocks would be 1.75 x 28 = 49 amps so maximum is 50 amp time delay fuses for 5 hp motor. You can go with less if you want as long as they do not blow when the motor starts.

For the feeder

The feeder conductor size is (28 x 1.25) + 28 = 63 amps this would be a #6 awg thwn if 75C terminations or #4 awg if 60C terminations.


Feeder over current protection would be 50A + 28 A = 78 amps ..must chose the next size down for a feeder so a 70 amp fuse maximum for feeder protection. You can use a 60 amp since that is your largest you say you can install.

Ground wire for the feeder can be # 10 copper

Conduit size feeder

3 #6 awg with a #10 ground in liquid tite = trade size 3/4" minimum suggest increase one trade size

Same for sch.40 pvc

For any areas above ground exposed to physical damage you need sch. 80 pvc this will require 1" sch.80

I wouldn't use the lfnc to go underground but it can be listed for that. It would be better to use sch. 40 pvc for that portion of the run.

For 3 #4awg thwn + #10 ground use minimum 1 " for all raceways
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Last edited by Stubbie; 08-06-2010 at 03:17 PM. Reason: Forgot #4 awg data
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Old 08-06-2010, 03:03 PM   #14
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I do not know the temp rating of the main 60 amp fuse holder for the feeder so I will use the 60 c and #4 wire. is the ground still#10 and how big of conduit?

Last edited by WDR; 08-06-2010 at 03:10 PM. Reason: mistake
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Old 08-06-2010, 03:14 PM   #15
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wire size for conduit run


I just edited my last post as I forgot to include the data for #4 ... sorry. Check bottom of my last post

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